1. Field of the Invention
The invention relates to a press roll which together with a mating roll defines a roll nip for the treatment of a moving web of material such as for dewatering of a web of paper in a paper machine.
2. Description of Related Technology
The press roll of the invention is based in part upon pressing apparatus which are known in the art, such as a compression roller for paper producing machinery disclosed in German patent publication 25 02 161 C, corresponding to Wolf, U.S. Pat. No. 4,064,607 (Dec. 27, 1977). These known apparatus typically include a press roll and a mating roll defining a roll nip. Such an apparatus may be oriented so that the median axes of the press roll and the mating roll lie in a non-vertical pressing plane and at least one of the two rolls is a "shoe roll."
A "shoe roll" may include a hollow, rotatable metallic roll casing disposed adjacent to the roll nip, a stationary yoke located within the roll casing and a shoe which transmits a pressing force from the yoke to the roll casing. Between the yoke and the shoe a pressure chamber may be provided in order to receive a pressure medium, such as hydraulic liquid for pressing the shoe against the roll casing. A lubricant film is also typically provided between the shoe and the roll casing in order to prevent metallic contact therebetween.
A consequence of providing a press roll with a hollow roll casing is that the roll casing flexes or bends under the action of the pressing force. The amount of bending preferably should be controlled so that variations in the pressing force along the width of a web of material being treated (i.e. along the length of the roll casing) is controlled as well. For example, if the press roll is mounted on a paper machine, the amount of bending of a roll casing should be controlled in order to provide a constant pressing force along the width of a web of paper being conveyed between the press roll and the mating roll.
In the compression roller disclosed in German patent publication 25 02 161 C and U.S. Pat. No. 4,064,607, it is assumed that the pressing plane (i.e. the plane containing the median axes of the press roll and the mating roll) is vertical. However, for the "shoe" type press roll according to the invention, a non-vertical pressing plane is assumed. For example, the pressing plane may be horizontal. When the pressing plane is not vertical, problems may arise because the pull of gravity on the press roll and the mating roll is not in the pressing plane and is rather at an angle thereto. Therefore, the lines of flexure resulting from the effect of gravity in the roll casing and the yoke may not be in agreement with each other. More particularly, the gravitational pull at the approximate center of the press roll in a median plane called the "shoe plane" (which is perpendicular to the pressing plane) may result in a maximum bend or flexure of the roll casing which is not equal to a maximum bend of flexure of the yoke. Because the shoe necessarily bends along its longitudinal axis in an amount approximately equal to the flexure or bend of the yoke, there may result a misfit between the outer surface of the shoe and the inner surface of the roll casing. As a result of this misfit there is a danger that the shoe may come into direct contact with some points on the inner surface of the roll casing which would cause an increase in frictional forces and wear.